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Eye On Electronics

Tire pressure monitoring systems are designed to solve the problem of underinflated tires. But, Mike tells us, current monitoring equipment might not be up to speed.

It's no secret to shop
owners and technicians: The average motorist doesn't check the air
pressure in his car's tires nearly often enough.
Even for those who do, the number of service stations that have free air
and accurate gauges has been dwindling as the number of gasoline/
convenience stores has expanded. Underinflation gets most of the blame
for the serious problem of last year, which led to the recall of 6
million Firestone tires. It's believed that those tires failed due to
the high heat generated by excessive sidewall flexing caused by
underinflation.

This is not just a Firestone problem, though. A survey conducted by Tire
Business showed that of 766 vehicles surveyed, 72% were operating with
at least one tire underinflated by an average of 6 psi; 40% of the
vehicles had all four tires underinflated by an average 7 psi. The
Rubber Manufacturers Association reported that in 1999, there were 647
tire-related fatalities in the U.S.

The Cycloid Corp. (Cranberry, Pennsylvania) makes tire pressure
monitoring systems (TPMS). By their numbers, underinflated tires waste
1.25 billion gallons of gasoline each year. According to the company,
low tire pressure is the leading cause of early wear-out of some 4.5
million tires a year. They say tire pressure monitoring could add as
much as half a mile per gallon to the CAFE rating of a vehicle. The
point is that underinflated tires are costly, dangerous and,
unfortunately, plentiful.

Well, the government has decided to do something about this. Starting in
November 2003, the National Highway Traffic Safety Administration (NHTSA)
will require 10% of 2004 model vehicles to have built-in tire pressure
monitoring systems. This figure rises to 35% of the 2005s, 65% of 2006
models and all of the 2007s. These new requirements are the result of a
mandate from Congress stemming from the Firestone fiasco.

First of all, what's really new here is that the whole fleet is going to
get tire pressure monitoring systems eventually. A number of
manufacturers have had these systems on high-end vehicles for a while.
GM, for example, has sold more than 2 million vehicles with these
systems over the last 15 years. High-end European and Japanese cars also
have variations of these systems. This NHTSA regulation will force them
into lower-price vehicles, as well.

Before deciding what sort of system carmakers will have to adopt, NHTSA
did an extensive study to determine what the capabilities and flaws were
in existing systems. Here is some of what they found out:
There are two basic kinds of TPMSs-pressure sensor-based (PSB) and wheel
speed-based (WSB). In the PSB direct-reading kind, a sensor and radio
transmitter are mounted in the tire, usually as a part of the valve
stem. A small battery inside the sensor powers the transmitter.
Tire pressure and temperature are broadcast to a receiver that then
relays the information to the vehicle's body computer. Because the
transmitters operate at around 500 mHz, they act like a cell phone and
are not bothered by the rotation of the wheel or the presence of metal
suspension and body parts. Because the distance the signal from each
transmitter has to cross is small, a lithium battery can provide the
power.

The wheel-speed-based (WSB) or indirect method of monitoring tire
pressure uses the wheel signal from the ABS sensor. As the tire's
inflation decreases, so, too, does its radius. The tire then has to
speed up to keep up with the other tires. Since most late-model vehicles
already have ABS and a body computer, the main cost of adding this
system is the need for a display device on the dash to give the
warnings. NHTSA estimates the manufacturing cost to be in the area of
$13. In contrast, the direct-reading system is expected to add as much
as $200 to the cost of a vehicle.

NHTSA tested four WSB systems. All could detect a tire that was 50% low
on inflation, provided the car was on a winding road course. On a
7.5-mile oval track, a situation where there's very little turning input
from the driver, only three of the four systems could detect a tire with
only 14 psi in it! NHTSA concluded that the wheels on the same axle
needed to be traveling at different rotational speeds for the WSB
software to make a clean detection.

In the NHTSA test, none of the four WSB systems was able to warn if all
four tires were underinflated. Remember, there has to be a speed
difference between any single tire vs. the other three for the WSB
concept to work.

There are other issues with WSB-type systems, as well. NHTSA's report
says that "The wheel-speed-based systems must be reset if the inflation
in one or more tires is changed, if one or more tires is replaced or
repaired or if the tires are rotated. The reset procedure allows the WSB
TPMS to �relearn' the rotational signature of each tire."

According to NHTSA, based on expected tire life and service intervals,
these reset procedures could need to be done as many as a hundred times
during the life of a vehicle. The kicker is that with the systems
tested, it was possible for someone to reset the system without having
had the tire inflations corrected.

The NHTSA study concluded that, in general, WSB systems have trouble
detecting more than one low tire at a time. A tire's circumference
doesn't change very much over the range of 15 to 40 psi, so WSB systems
generally require a drop of between 10% and 40% before a warning is
issued. These warning thresholds cannot easily be changed by the
manufacturer, since they're highly dependent on tire characteristics,
driving conditions and load factors.

One good thing they found is that as tires heat due to low inflation,
the pressure increase inside the tire is not enough to turn off the
warning system. Also at issue is ambient temperature. You don't want the
system to give nuisance warnings when what's really being sensed is the
result of cold weather.

Testing of various manufacturers' pressure sensor-based systems showed
that the average underinflation warning occurred at levels 20% below the
recommended cold fill pressure. A second, more serious warning was given
at 36% underinflation. Tests to explore temperature effects on PSB
systems showed that those systems without temperature compensation had
less than a 1-psi change in their warning threshold levels for a change
of 30� to 50�F ambient. Further testing of PSB systems showed that,
generally, they're accurate to within 1 to 2 psi.

One very interesting test that was run was to see how the PSB system
would react to a sudden loss of pressure. For this test, a car was
parked, four valve stems were opened and the time until the driver was
warned was noted. The average notification time was more than a minute.
Some systems warned in 8 seconds, but one took 136 seconds to issue the
warning. It's thought that some systems update more often than others.
The result is that some systems might be useful for blowout detection,
while others may give no useful information at all.

In general, NHTSA concluded, the pressure sensor-based systems were able
to detect any combination of one or more underinflated tires. The
warning thresholds were consistent from tire to tire and can easily be
changed by the manufacturer or the vehicle owner. Some of the tested PSB
systems have automatic sensor location recognition. This eliminates the
need to "retrain" the system after a wheel rotation.

Nothing is ever perfect, including PSB monitoring systems. Beyond the
issue of costing 15 times as much as the WSB systems, PSB systems
require additional hardware. As noted, there's a transmitter in every
wheel that acts as mass or weight that has to be balanced in the tire
shop. While the batteries are supposed to last five to ten years, it'll
be an unpleasant expense for the owner to have to have the tires pulled
off and then rebalanced to replace a failed battery.

To determine the effects of a sensor failure, a test was set up where
the left front wheel sensor was removed from each of six vehicles using
different makes of PSB systems. Two of the six warned of a sensor
problem on startup. One indicated zero pressure at startup, but after 14
minutes of operation, it warned of a sensor error. A fourth system
warned of a sensor failure, but only after the driver pressed the Status
button. The remaining systems reported zero pressure, but did not
indicate that there was a pressure sensor problem.

Other manufacturers have tried mounting the hardware externally, but
this exposes it to various road debris, salt and other hazards. There
has been concern voiced about what happens when someone uses one of
those roadside tire-inflation products. Since most of the PSB systems
are constructed as part of the tire valve, such a leak-stopping compound
could clog the sensor. Another worry is tire removal and replacement.
The guys in the tire shop will have to be very careful not to damage the
PSB hardware when removing or installing tires on wheels.

As mentioned, there has been a lot of discussion over how good these
systems need to be. A mandate that every new car needs a $200 tire
pressure monitoring system will stir up a lot of opposition. As is
normal with agencies like NHTSA, a preliminary ruling is issued as to
what the requirements are going to be. Then they ask for comment from
concerned parties before making the final decision. NHTSA got lots of
comments on its first proposal that included a requirement that a driver
had to be warned when any given tire was 25% or more underinflated.

The Rubber Manufacturers Association pointed out that a tire that's 25%
underinflated might not be safe to drive on. The pressure requirement
placed on the side of the tire is the minimum cold pressure needed to
support the maximum safe load of the vehicle. The concern is that if a
tire pressure monitoring system doesn't go off, a driver will think the
tire is sufficiently inflated.

In the end, NHTSA issued its final rules in July 2002, and they're a
compromise. Vehicle manufacturers have two options-one, that the TPMS
must warn the driver when the pressure in any single tire or combination
of tires has fallen more than 25%; or two, that the warning must be
issued when any single tire falls 30% or more below the recommended cold
inflation pressure.

So either system can be used over the next three years of the phase-in.
During this time, studies will be conducted and further system
development will occur...one hopes. At the end of the three years, NHTSA
will look at the tire pressure monitoring issue again and decide what
the permanent long-term warning requirement is going to be.

The thing you want to remember and pass along to your customers is that
tire pressure monitoring systems are going to trigger only when there's
a major underinflation. The lost fuel economy, early wear-out of tires,
loss of braking and handling performance and those sorts of things will
not be prevented by these monitoring systems. Remind your customers that
they still need to monitor the pressure in their tires.